Answer :
Final answer:
In this isolated system, the net force acting on the girl and the plank is zero. This is because according to Newton's third law, the force exerted by the girl on the plank is offset by the force exerted by the plank on the girl.
Explanation:
The situation described in the question involves a girl moving on a plank on a frictionless surface - essentially this is an isolated system. According to Newton's third law - for every action, there is an equal and opposite reaction. If the girl walks to the right, she exerts a force on the plank to the left. Conversely, the plank exerts an equal force on the girl to the right. However, the net force in the girl-plank system is zero because these forces cancel each other out.
This is due to the principle of conservation of momentum in isolated systems. Since there are no external forces in this horizontal system, the total momentum of the system (girl and the plank) remains constant. Therefore, even as the girl moves to one side of the plank, the plank moves in the opposite direction but their collective motion keeps the system at rest, preserving the total momentum.
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